Republican presidential candidate Marco Rubio raised alarm about the threat of terrorists illegally crossing the U.S.-Mexican border and called for a tougher U.S. immigration system during a campaign event in New Hampshire on Thursday.

'ISIS Exploiting Loopholes'

While speaking at a packed rally in Bedford, the Florida senator stated that Islamic State terrorists are exploiting loopholes in the U.S. immigration system and sending jihadists into the country to kill Americans.

"As someone who has access to very sensitive information, I can tell you for a fact that ISIS understands our legal immigration system and is seeking to exploit it," said Rubio, who is a member of the Senate Intelligence Committee, according to Bloomberg Politics.

The 2016 hopeful also used the recent ISIS-inspired terrorist attacks in San Bernardino and Paris as reasons why to strengthen the nation's immigration laws.

"We now have radical jihadist groups that are using our immigration system against us. They are trying to get people into this country as doctors, as students, as investors, as tourists. They've already gotten someone into this country as a fiancée," he said referring to San Bernardino shooters.

"They understand that our southern border is weak, and they're trying to exploit that as well. They are radical jihadist groups. The same people who inspired the attacks in Paris, who inspired the attacks in California are trying to use our immigration system against us," he added.

More Secure Border, Tighter Visa Requirements

In addition to calling for more border security, Rubio also talked about the need to tighten a visa waiver program.

About an hour later, he presented the same message while delivering a speech at Nashua Community College.

"When I am president, our immigration system is going to be guided by a very simple principle. If we don't know 100 percent for sure who you are, and 100 percent for sure why you are coming, you will not be allowed to enter the United States of America," he said. "We're not going to have amnesty."

Although his message appeared to be widely received by attendees, pro-immigration advocates bashed Rubio over his new hardline stance towards immigration. Frank Sharry, the executive director of America's Voice, pointed out that the Cuban American candidate was once a champion of comprehensive reform measures that would have provided undocumented residents with a pathway to citizenship.

"His fall from grace is complete," said Sharry. "In 2013, he became a courageous champion of immigration reform and now he's lurching to the right almost as fast as Ted Cruz. Once hailed as the guy who would save the GOP from its extremist wing, he's now pandering to it."